This invention relates to attenuators for altering the amplitude of an electrical input signal and, more particularly, to a distributed network resistive film attenuator. Specifically, one embodiment of the invention provides a resistive film attenuator element comprised of a dielectric-mounted resistive film distributed ladder network having tuning stubs, combined in a coplanar structure, to provide an attenuator having a substantially flat frequency response over a wide range of frequencies, for example, from D.C. to 40 GHz.
A distributed network resistive film attenuator is described in U.S. Pat. No. 3,227,975 issued to Hewlett-Packard Company and entitled Fixed Coaxial Line Attenuator with Dielectric Mounted Resistive Film. This attenuator has a substantially constant attenuation over a wide range of frequencies, for example, from D.C. to 18 GHz.
Considered in more detail, U.S. Pat. No. 3,227,975 discloses a fixed coaxial attenuator comprising a dielectric plate supported within a cylindrical outer conductor between sections of a coaxial inner conductor. A rectangular sheet of resistive material having a predetermined width and a predetermined length is positioned on the dielectric plate between first and second pairs of electrodes. The first pair of electrodes provides electrical contacts between the outer conductor and the lengthwise sides of the rectangular sheet along the full length thereof. The second pair of electrodes provides electrical contacts between the sections of the coaxial inner conductor and a central portion of the lateral sides of the rectangular sheet.
Resistive film attenuators of this type suffer from several known limitations. Most importantly, in order to achieve a desired attenuation or a desired impedance throughout the operating frequency range of the attenuator, the resistive film may have to be made long in order to maintain a desired attenuation and impedance, and this may affect attenuation characteristics at higher frequencies.
Furthermore, distributed network resistive film attenuators are also incorporated into cascade attenuators of the type described in U.S. Pat. No. 3,319,194 issued to Hewlett-Packard Company and entitled Variable Attenuator Employing Internal Switching. This high frequency signal attenuator provides discrete steps of attenuation using separate attenuator elements which are all disposed in a transmission line configuration adjacent a common and continuous ground plane. An attenuator of this type obviates the need for complex mechanisms for switching both the signal and ground plane conductors of the transmission line structure and thus eliminates the introduction of unknown contact impedances in the ground plane conductor at the junctions of attenuator sections.
Considered in more detail, the step attenuator for high frequency signals disclosed in U.S. Pat. No. 3,319,194 comprises a strip line structure formed in a continuous ground plane conductor using a number of switchable sections, each including a resistive card attenuator and a straight-through conductor. Selection of either of the two signal paths is accomplished by deflecting the signal conductor from contact with one signal path to contact with the other signal path using magnetic or mechanical actuators.
In both the case of the fixed coaxial line attenuator disclosed in U.S. Pat. No. 3,227,975 and in the case of the resistive card attenuators included in the cascade attenuator disclosed in U.S. Pat No. 3,319,194, different values of attenuation in nepers may be selected by altering the length of resistive film. This is especially difficult in a cascade attenuator wherein changes in the lengths of the resistive card attenuators, vis-a-vis the lengths of the straight-through conductive elements, can adversely affect the alignment of the resistive card attenuators with the switches and degrade the quality of electrical connection when the resistive card attenuators are switched in and out of the electrical circuit.
Accordingly, U.S. Pat. No. 3,521,201, also issued to Hewlett-Packard Company and entitled Coaxial Attenuators Having at Least Two Regions of Resistive Material, discloses a distributed network resistive film attenuator having a substantially constant attenuation over a broad frequency range comprised of two aligned rectangular areas of resistive film disposed a selected distance apart on a substrate supported within an outer coaxial conductor, each area having small aligned rectangular apertures therein to provide selected values of resistivity per unit area within selected portions of the film. The resistive film areas are connected by a connecting electrode of a selected length which is less than one-half of the wavelength of the highest frequency electromagnetic wave energy being attenuated to prevent resonance. A first pair of electrodes provides electrical contacts between the outer conductor and opposite edges of both rectangular areas of resistive film, and a second pair of electrodes provides electrical contacts between sections of a coaxial inner conductor and the resistive film areas, thereby interconnecting both areas between the coaxial inner conductor sections.
U.S. Pat. No. 3,521,201 discloses that the shape and location of the apertures within the resistive film determine resistivity per unit area of the film. By providing aligned equally-spaced rectangular apertures of different length and width dimensions, the resistivity per unit area can be varied along a selected direction in the plane of the resistive film. The portions of the resistive films having square apertures provide in effect a series resistance between inner conductor sections, while the portions of the resistive films having rectangular apertures therein provide in effect a shunt resistance between the central portion of the resistive films and outer conductor. Other patterns of apertures may be used to provide logarithmic or exponential or other desired variations with length in the resistivity per unit area of the resistive film. The disclosed apertures are rectangular holes and square holes disposed in a grid pattern on a substrate, but, in general, these apertures may have any shape or be arranged in any suitable pattern which provides the required resistivity per unit area of the resistive films. The desired values of resistivity per unit area in these portions of the resistive films may thus be obtained by selectively varying the size, shape, and spacing of the apertures. It is readily apparent that this technique to provide the desired resistivity per unit area is quite complex.
Also, U.S. Pat. No. 3,521,201 discloses that the length of the connecting electrode is selected for greatest linearity of attenuation with frequency over a broad frequency range from D.C. to about 18 GHz. Specifically, the connecting electrode is not longer than one-half of a wavelength at the highest operating frequency of the attenuator. Signal delay along the length of the connecting electrode between the two, otherwise isolated, resistive sheets improves the linearity with frequency of the attenuation at frequencies from about 12.4 GHz to about 18 GHz. It is readily apparent that fabrication of the two resistive film areas connected by a connecting electrode also adds to manufacturing complexity.
In view of the structural complexity of the type of resistive film attenuator disclosed in U.S. Pat. No. 3,521,201, the structure of resistive film attenuators has evolved to the configuration shown in FIG. 1. The resistive film attenuator shown in FIG. 1 comprises a resistive film distributed ladder network having resistive film 1 patterned on a dielectric material 2. The respective ends of the series resistive film portions 1A are connected to respective contacts 4 that interconnect to respective inner coaxial contacts, and the shunt resistive film portions 1B are connected to respective contacts 3 that interconnect to a coaxial outer conductor or opposing walls of a ground plane housing. Unfortunately, the flatness of the frequency response of this resistive film attenuator is controlled by changing the separation of the contacts 4. As in the case of the resistive film attenuator disclosed in U.S. Pat. No. 3,319,194, however, this is especially difficult in a cascade attenuator wherein changes in the lengths of the resistive card attenuators, vis-a-vis the lengths of the straight-through conductive elements, can adversely affect the alignment of the resistive card attenuators with the stitches and degrade the quality of electrical connection when the resistive card attenuators are switched in and out of the electrical circuit. Accordingly, there is a need for an economical, easily manufactured resistive film attenuator which has readily controllable values of attenuation and flat frequency response over a broad range of frequencies.